c++: Implement __add_pointer built-in trait

c++: Implement __add_pointer built-in trait

This patch implements built-in trait for std::add_pointer.

gcc/cp/ChangeLog:

	* cp-trait.def: Define __add_pointer.
	* semantics.cc (finish_trait_type): Handle CPTK_ADD_POINTER.
	(object_type_p): New function.
	(referenceable_type_p): Likewise.
	(trait_expr_value): Use object_type_p.

gcc/testsuite/ChangeLog:

	* g++.dg/ext/has-builtin-1.C: Test existence of __add_pointer.
	* g++.dg/ext/add_pointer.C: New test.

Signed-off-by: Ken Matsui <kmatsui@gcc.gnu.org>
Reviewed-by: Jason Merrill <jason@redhat.com>